Light source for lithography exposure process

ABSTRACT

A method for generating light is provided. The method further includes measuring a period of time during which one of targets from a fuel target generator passes through two detection positions. The method also includes exciting the targets with a laser generator so as to generate plasma that emits light. In addition, the operation of exciting the targets with the laser generator includes: irradiating a pre-pulse laser on the targets to expand the targets; detecting conditions of expanded targets; and adjusting at least one parameter of the laser generator according to the measured period of time and the conditions when the measured period of time is different from a predetermined value. The parameter of the laser generator which is adjusted according to the measured period of time includes a frequency for generating a laser for illuminating the targets.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is Continuation application of U.S. patent applicationSer. No. 16/671,347, filed on Nov. 1, 2019, now U.S. Pat. No.10,993,308, which is a Continuation Application of application Ser. No.15/868,373, filed on Jan. 11, 2018, now U.S. Pat. No. 10,477,663, whichclaims the benefit of U.S. Provisional Application No. 62/586,992, filedon Nov. 16, 2017, the entirety of which is incorporated by referenceherein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometric size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling-down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling-down has also increased the complexity ofprocessing and manufacturing ICs.

For example, there is a growing need to perform higher-resolutionlithography processes. One lithography technique is extreme ultravioletlithography (EUVL). The EUVL employs scanners using light in the extremeultraviolet (EUV) region, having a wavelength of about 1-100 nm. Onetype of EUV light source is laser-produced plasma (LPP). LPP technologyproduces EUV light by focusing a high-power laser beam onto small fueldroplet targets to form highly ionized plasma that emits EUV radiationwith a peak of maximum emission at 13.5 nm. The EUV light is thencollected by a collector and reflected by optics towards a lithographyexposure object, e.g., a wafer.

Although existing methods and devices for generating EUV light have beenadequate for their intended purposes, they have not been entirelysatisfactory in all respects. Consequently, it would be desirable toprovide a solution for increasing power conversion efficiency from theinput energy for ionization.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 shows a schematic view of a lithography system with a lightsource, in accordance with some embodiments.

FIG. 2 is a diagrammatic view of the light source in the lithographysystem of FIG. 1, in accordance with some embodiments.

FIG. 3 is a diagrammatic view of partial elements of the light sourcewhile targets are generated by a fuel target generator, in accordancewith some embodiments.

FIG. 4 is a flowchart of a method for a generating light, in accordancewith some embodiments.

FIG. 5 is a diagram of a signal intensity detected by a detector versustime, in accordance with some embodiments.

FIG. 6 is a diagrammatic view of a light source in the lithographysystem, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the subject matterprovided. Specific examples of solutions and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Furthermore, spatially relative terms, such as “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. It should be understoodthat additional operations can be provided before, during, and after themethod, and some of the operations described can be replaced oreliminated for other embodiments of the method.

The advanced lithography process, method, and materials described in thecurrent disclosure can be used in many applications, including fin-typefield effect transistors (FinFETs). For example, the fins may bepatterned to produce a relatively close spacing between features, forwhich the above disclosure is well suited. In addition, spacers used informing fins of FinFETs can be processed according to the abovedisclosure.

FIG. 1 is a schematic and diagrammatic view of a lithography system 10,in accordance with some embodiments. The lithography system 10 may alsobe generically referred to as a scanner that is operable to performlithography exposing processes with respective radiation source andexposure mode.

The lithography system 10 includes a light source 12, an illuminator 14,a mask stage 16, a mask 18, a projection optics module (or projectionoptics box (POB)) 20 and a substrate stage 24, in accordance with someembodiments. The elements of the lithography system 10 can be added toor omitted, and the invention should not be limited by the embodiment.

The light source 12 is configured to generate radians having awavelength ranging between about 1 nm and about 100 nm. In oneparticular example, the light source 12 generates an EUV light with awavelength centered at about 13.5 nm. Accordingly, the light source 12is also referred to as an EUV light source. However, it should beappreciated that the light source 12 should not be limited to emittingEUV light. The light source 12 can be utilized to perform anyhigh-intensity photon emission from excited target material.

In various embodiments, the illuminator 14 includes various refractiveoptic components, such as a single lens or a lens system having multiplelenses (zone plates) or alternatively reflective optics (for EUVlithography system), such as a single mirror or a mirror system havingmultiple mirrors in order to direct light from the light source 12 ontoa mask stage 16, particularly to a mask 18 secured on the mask stage 16.In the present embodiment where the light source 12 generates light inthe EUV wavelength range, reflective optics is employed.

The mask stage 16 is configured to secure the mask 18. In someembodiments, the mask stage 16 includes an electrostatic chuck (e-chuck)to secure the mask 18. This is because the gas molecules absorb EUVlight and the lithography system for the EUV lithography patterning ismaintained in a vacuum environment to avoid EUV intensity loss. In thepresent disclosure, the terms mask, photomask, and reticle are usedinterchangeably.

In the present embodiment, the mask 18 is a reflective mask. Oneexemplary structure of the mask 18 includes a substrate with a suitablematerial, such as a low thermal expansion material (LTEM) or fusedquartz. In various examples, the LTEM includes TiO2 doped SiO2, or othersuitable materials with low thermal expansion. The mask 18 includesreflective multilayer deposited on the substrate.

The reflective multilayer includes a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair). Alternatively, thereflective multilayer may include molybdenum-beryllium (Mo/Be) filmpairs, or other suitable materials that are configurable to highlyreflect the EUV light. The mask 18 may further include a capping layer,such as ruthenium (Ru), disposed on the reflective multilayer forprotection. The mask 18 further includes an absorption layer, such as atantalum boron nitride (TaBN) layer, deposited over the reflectivemultilayer. The absorption layer is patterned to define a layer of anintegrated circuit (IC). Alternatively, another reflective layer may bedeposited over the reflective multilayer and is patterned to define alayer of an integrated circuit, thereby forming an EUV phase shift mask.

The projection optics module (or projection optics box (POB)) 20 isconfigured for imaging the pattern of the mask 18 on to a semiconductorwafer 22 secured on a substrate stage 24 of the lithography system 10.In some embodiments, the POB 20 has refractive optics (such as for a UVlithography system) or alternatively reflective optics (such as for anEUV lithography system) in various embodiments. The light directed fromthe mask 18, carrying the image of the pattern defined on the mask, iscollected by the POB 20. The illuminator 14 and the POB 20 arecollectively referred to as an optical module of the lithography system10.

In the present embodiment, the semiconductor wafer 22 may be made ofsilicon or other semiconductor materials. Alternatively or additionally,the semiconductor wafer 22 may include other elementary semiconductormaterials such as germanium (Ge). In some embodiments, the semiconductorwafer 22 is made of a compound semiconductor such as silicon carbide(SiC), gallium arsenic (GaAs), indium arsenide (InAs), or indiumphosphide (InP). In some embodiments, the semiconductor wafer 22 is madeof an alloy semiconductor such as silicon germanium (SiGe), silicongermanium carbide (SiGeC), gallium arsenic phosphide (GaAsP), or galliumindium phosphide (GaInP). In some other embodiments, the semiconductorwafer 22 may be a silicon-on-insulator (SOI) or a germanium-on-insulator(GOI) substrate.

In addition, the semiconductor wafer 22 may have various deviceelements. Examples of device elements that are formed in thesemiconductor wafer 22 include transistors (e.g., metal oxidesemiconductor field effect transistors (MOSFET), complementary metaloxide semiconductor (CMOS) transistors, bipolar junction transistors(BJT), high voltage transistors, high-frequency transistors, p-channeland/or n-channel field-effect transistors (PFETs/NFETs), etc.), diodes,and/or other applicable elements. Various processes are performed toform the device elements, such as deposition, etching, implantation,photolithography, annealing, and/or other suitable processes. In someembodiments, the semiconductor wafer 22 is coated with a resist layersensitive to the EUV light in the present embodiment. Various componentsincluding those described above are integrated together and are operableto perform lithography exposing processes.

The lithography system 10 may further include other modules or beintegrated with (or be coupled with) other modules. In the presentembodiment, the lithography system 10 includes a gas supply module 26designed to provide hydrogen gas to the light source 12. The hydrogengas helps reduce contamination in the light source 12.

FIG. 2 illustrates the light source 12 in a diagrammatical view, inaccordance with some embodiments. The light source 12 employs adual-pulse laser produced plasma (LPP) mechanism to generate plasma andfurther generate EUV light from the plasma.

In some embodiments, the light source 12 includes a controller 13, afuel target generator 30, a laser generator LG, a laser produced plasma(LPP) collector 60 and a monitoring device 70. The above-mentionedelements of the light source 12 may be held under vacuum. It should beappreciated that the elements of the light source 12 can be added to oromitted, and the invention should not be limited by the embodiment.

The fuel target generator 30 is configured to generate a plurality oftargets 82. In some embodiments, the fuel target generator 30 includes avessel 31 for containing a target material (not shown in figures) and agas supplier 32. The gas supplier 32 is connected to the vessel 31 andconfigured to supply a pumping gas 33 into the vessel 31. The pumpinggas 33 increases the pressure in vessel 31 so as to force targetmaterial contained in the vessel 31 out of the fuel target generator 30and drive the flowing of the targets 82.

In some embodiments, a flow velocity of the targets 82 from the fueltarget generator 30 is a function of the pressure of the pumping gas 33in the fuel target generator 30. For example, the targets 82 flow fasterwhen the pressure of the pumping gas 33 in the vessel 31 is increased,and the targets 82 flow slower when the pressure of the pumping gas 33in the vessel 31 is reduced.

In some embodiments, the fuel target generator 30 further includes anozzle 34 and an actuator 35. The nozzle 34 is connected to the vessel31 for dispensing the targets 82, and the actuator 35 surrounds thenozzle 34. The actuator 35, for example, includes a piezoelecticmaterial. The actuator 35 applies a force on the nozzle 34 in responseto control signal from the controller 13 so as to supply the targets 82with a predetermined mode.

For example, the controller 13 supplies a voltage to the actuator 35 ata given frequency, causing the actuator 35 to press the nozzle 34 at thetime of receiving the voltage and stop the pressing when no voltage isreceived. As a result, the nozzle 34 may supply a plurality of targets82 in the form of micro-droplets into the excitation zone 81. In someother embodiments, a droplet pitch between two neighboring targets 82and/or the diameter of the targets 82 are controlled as a function ofthe frequency at which the voltage is supplied to the actuator 35.

In an embodiment, the targets 82 are tin (Sn) droplets. In anembodiment, the targets 82 each have a diameter about 30 microns (μm).In an embodiment, the targets 82 are generated at a rate about 50kilohertz (kHz) and are introduced into a zone of excitation 81 in thelight source 12 at a speed about 70 meters per second (m/s). Othermaterial can also be used for the targets 82, for example, a tincontaining liquid material such as eutectic alloy containing tin,lithium (Li), and xenon (Xe). The targets 82 may be in a solid or liquidphase.

The laser generator LG is configured to generate at least one laser toallow a conversion of the targets 82 into plasma. In some embodiments,the laser generator LG includes a first laser source 40 and a secondlaser source 50. The first laser source 40 is configured to produce apre-pulse laser 42. The second laser source 50 is configured to producea main pulse laser 52. The pre-pulse laser 42 is used to heat (orpre-heat) the targets 82 to expand the targets 82, which is subsequentlyirradiated by the main pulse laser 52, generating increased emission oflight.

In an embodiment, the first laser source 40 is a carbon dioxide (CO2)laser source. In another embodiment, the first laser source 40 is aneodymium-doped yttrium aluminum garnet (Nd:YAG) laser source. In anembodiment, the second laser source 50 is a CO2 laser source.

In the present embodiment, the pre-pulse laser 42 has less intensity anda smaller spot size than the main pulse laser 52. In variousembodiments, the pre-pulse laser 42 has a spot size of about 100 μm orless, and the main pulse laser 52 has a spot size about 200-300 μm, suchas 225 μm. The pre-pulse laser 42 and the main pulse laser 52 aregenerated to have certain driving powers to fulfill wafer volumeproduction, such as a throughput of 125 wafers per hour. For example,the pre-pulse laser 42 is equipped with about 2 kilowatts (kW) drivingpower, and the main pulse laser 52 is equipped with about 19 kW drivingpower. In various embodiments, the total driving power of the pre-pulselaser 42 and the main pulse laser 52, is at least 20 kW, such as 27 kW.However, it should be appreciated that many variations and modificationscan be made to embodiments of the disclosure.

The pre-pulse laser 42 and main pulse laser 52 are directed throughwindows (or lenses) 44 and 54, respectively, into the zone of excitation81 and irradiate targets 82 at a first lighting position LP1 and asecond lighting position LP2. The windows 44 and 54 adopt a suitablematerial substantially transparent to the respective laser beams. Themethod for exciting the targets 82 supplied by the fuel target generator30 is described later.

The monitoring device 70 is configured to monitor conditions of thetargets 82 supplied by the fuel target generator 30. In someembodiments, the monitoring device 70 includes a transducer 71 and adetector 72. The transducer 71 is configured to generate one or moredetection signals for monitoring conditions of the targets 82, and thedetector 72 is configured to receive the detection signal reflected bythe targets 82.

In some embodiments, as shown in FIG. 3, the transducer 71 includes twolight emitters, such as first light emitter 711 and second light emitter712. The first light emitter 711 continuously generates a first lightcurtain L1 during the supply of the targets 82. The second light emitter712 continuously generates a second light curtain L2 during the supplyof the targets 82. The first light emitter 711 and the second lightemitter 712 are arranged so that a first detection position DP1 and asecond detection position DP2 are illuminated by the first light curtainL1 and the second light curtain L2. The light curtain L1 and the lightcurtain L2 may each include a number of light beams arrangedsequentially along a straight line or curved line. In some otherembodiments, each of the first light emitter 711 and the second lightemitter 712 generates a single light beam to a respective one of thefirst detection position DP1 and the second detection position DP2.

In some embodiments, the first detection position DP1 and the seconddetection position DP2 are arranged on a moving path 85 along which thetargets 82 are moved. The first detection position DP1 and the seconddetection position DP2 are spaced apart by a distance D. The distance Dis smaller than a droplet pitch P between two neighboring targets 82.The ratio between the distance D and the droplet pitch P may be about0.1 to about 0.9. The ratio between the distance D and the droplet pitchP may be about 0.3 to about 0.5. The light curtain L1 and the lightcurtain L2 may each include a number of light beams arrangedsequentially along a straight line or curved line. In some otherembodiments, each of the first light emitter 711 and the second lightemitter 712 continuously emits one single light beam.

The first light emitter 711 and the second light emitter 712 may emitradiation such as laser having wavelength about 1070 nm. The drivingpower of the radiation emitted by the first light emitter 711 and thesecond light emitter 712 is sufficiently less than the driving power ofthe pre-pulse laser 42 and the main pulse laser 52. The radiation fromthe first light emitter 711 and the radiation from the second lightemitter 712 may be different or the same. For example, the wavelength ofradiation emitted from the first light emitter 711 is higher than thewavelength of the radiation from the second light emitter 712 forincreasing detection accuracy.

The detector 72 is arranged adjacent to the transducer 71 so as toreceive light reflected from the first detection position DP1 and thesecond detection position DP2 when an individual target 82 passesthrough the first detection position DP1 and the second detectionposition DP2. In some embodiments, the front surface 721 of the detector72 that is used for receiving the signals is not parallel to the movingpath 85 and is oriented toward the fuel target generator 30. The frontsurface 721 may be inclined relative to the moving path 85 so that thereflected first light curtain L1 and the reflected second light curtainL2 are perpendicularly projected on the front surface 721. As a result,the detected intensity of the first light curtain L1 and the secondlight curtain L2 is maximized and noise caused by the pre-pulse laser 42and the main pulse laser 52 is minimized.

By recording the time when the light reflected from the first detectionposition DP1 is detected and by recording the time when the lightreflected from the second detection position DP2 is detected, the periodof time during which an individual target 82 passes through the firstdetection position DP1 and the second detection position DP2 can bemeasured. In some embodiments, the detector 72 is electrically connectedto the controller 13. The measured result is transmitted to thecontroller 13 by the detector 72 for further processing.

It should be appreciated that while there is only one detector 72arranged for detecting the reflected light, many variations andmodifications can be made to embodiments of the disclosure. In someother embodiments, there are two detectors 72 are used to receive lightreflected from the first detection position DP1 and the second detectionposition DP2.

In addition, while the embodiment shown in FIG. 3 uses optical signals(e.g., laser) for measuring the period of time that targets pass twodetection positions, other technique can be utilized for measuring theperiod of time. For example, the monitoring device 70 may include acamera and an image analyzer. The camera is used to capture images ofthe targets 82 when they pass through the first detection position DP1and the second detection position DP2. With the recorded images, theperiod of time that the individual target passes two detection positionscan be measured by analyzing the images of the first detection positionDP1 and the second detection position DP2 with an image analyzer.

The controller 13 is configured to control one or more elements of thelight source 12. In some embodiments, the controller 13 is configured todrive the fuel target generator 30 to generate the targets 82. Inaddition, the controller 13 is configured to drive the first lasersource 40 and the second laser source 50 to fire the pre-pulse laser 42and the main pulse laser 52. The generation of the pre-pulse laser 42and the main pulse laser 52 may be controlled to be associated with thegeneration of targets 82 by the controller 13 so as to make thepre-pulse laser 42 and the main pulse laser 52 hit each target 82 insequence. Moreover, the controller 13 is configured to control thesupply of pumping gas 33 from the gas supplier 32 into the fuel targetgenerator 30 so as to change the flow velocity of the targets 82 fromthe fuel target generator 30.

The controller 13 may be a computer system. In one example, the computersystem includes a network communications device or a network computingdevice (for example, a mobile cellular phone, a laptop, a personalcomputer, a network server, etc.) capable of communicating with anetwork. In accordance with embodiments of the present disclosure, thecomputer system performs specific operations via a processor executingone or more sequences of one or more instructions contained in a systemmemory component.

The processor may include a digital signal processor (DSP), amicrocontroller (MCU), and a central processing unit (CPU). The systemmemory component may include a random access memory (RAM) or anotherdynamic storage device or read only memory (ROM) or other static storagedevices, for storing data and/or instructions to be executed by theprocessor. For example, the system memory component may store apredetermined value of a flow velocity of targets, a predetermined valueof the period of time during an individual target passé two detectionposition, or an acceptable range for adjusting parameter of the lasergenerator.

FIG. 4 is a flowchart of a method 90 for generating a light, inaccordance with some embodiments. For illustration, the flow chart willbe described along with the drawings shown in FIGS. 1-3 and 5. Some ofthe described transportation stages can be replaced or eliminated indifferent embodiments.

The method 90 includes operation 91, in which targets 82 are generatedby forcing the target material out of the droplet generator 30. In someembodiments, the fuel target generator 30 shown in FIG. 2 is configuredto generate the targets 82. The fuel target generator 30 is controlledto generate targets 82 with the proper material, proper size, properfrequency, and proper flow velocity and direction according to aprocessing recipe.

The method 90 also includes operation 92 in which the period of timeduring which one of the targets 82 passes through the first detectionposition DP1 and the second detection position DP2 is measured. In someembodiments, operation 92 is performed by the monitoring device 70. Asshown in FIG. 3, the first light curtain L1 and the second light curtainL2 from the transducer 71 continuously illuminate on the first detectionposition DP1 and the second detection position DP2, respectively.

When one of the targets 82 passes through the first detection positionDP1 at time t1, the first light curtain L1 is reflected by the target 82and detected by the detector 72. In addition, when the targets 82, asindicated by dotted line, passes through the second detection positionDP2 at time t2, the second light curtain L2 is reflected by the target82 and detected by the detector 72.

One embodiment of the detection result of the detector 72 is showndiagram of FIG. 5. A period of time Δt between time t1 and time t2 canbe measured by subtracting the time t2 by the time t1. In someembodiments, radiation from the pre-pulse laser 42 or the main pulselaser 52 is also detected by the detector 72 and produces signal at timet3. This noise signal may be mitigated by positioning the detector 72 ina proper manner, or by utilizing a filtering technique to ignore signalhaving intensity less than I_(min).

The method 90 also includes operation 93, a flow velocity of the targets82 is calculated. In some embodiments, the data associated the timedifference Δt is sent to the controller 13. The controller 13 calculatesflow velocity of the targets 82 by dividing the distance D (FIG. 3) bythe measured time difference Δt. Afterwards, the method 90 continueswith operation 94, in which the controller 13 determines if thecalculated flow velocity is different from a predetermined value of thepredetermined processing recipe.

When the calculated flow velocity is the same as the predetermined valueof the predetermined processing recipe, the method continues withoperation 95. In operation 95, the targets 82 are excited to generatelight. One method for exciting the targets 82, in accordance with someembodiments, is described below.

In the beginning, the first laser source 40 is used to generate thepre-pulse laser 42 to expand the targets 82 at the first lightingposition LP1. Before being irradiated by the pre-pulse laser 42, thetargets 82 have circular shape. After the targets 82 are irradiated bythe pre-pulse laser 42, a portion of the pre-pulse laser 42 is convertedto kinetic energy to transform the targets 82 to expanded targets 83with pancake-shape, as shown in FIG. 2.

Afterwards, the second laser source 50 is used to generate the mainpulse laser 52 to excite the expanded targets 83 at the second lightingposition LP2. The main pulse laser 52 heats the expanded targets 83 to apreset temperature. At the preset temperature, the target material 80 inthe expanded targets 83 shed their electrons and become a plasma thatemits light 84.

In some embodiments, the first laser source 40 is configured to generatethe pre-pulse laser 42 after a time interval when the detector 72receives the first light curtain L1. Because the targets 82 are moved asthe desired flow velocity and because an intermediate distance betweenthe first detection position DP1 and the first lighting position LP1 isfixed, the time interval can be calculated by dividing the intermediatedistance by the desired flow velocity. As a result, the targets 82 canbe accurately irradiated by the pre-pulse laser 42 when the targets 82reach the first lighting position LP1.

On the other hand, when the calculated flow velocity is different fromthe preset value of the predetermined processing recipe, the methodcontinues with operation 96. In operation 96, the parameter of the lasergenerator LG is adjusted. In some embodiments, the firing time or thefiring frequency of the pre-pulse laser 42 and the main pulse laser 52is adjusted by the controller 13 according to the calculated flowvelocity so as to accurately irradiate the targets 82 once the targets82 get the first lighting position LP1 and the second lighting positionLP2.

For example, when the calculated flow velocity is greater than thepredetermined value of the predetermined processing recipe, which meansmany more targets will pass through the first lighting position LP1 andthe second lighting position LP2, an increase in the firing frequencymay allow most of the targets 82 to be irradiated by the pre-pulse laser42 and the main pulse laser 52. Conversely, when the calculated flowvelocity is less than the predetermined value of the predeterminedprocessing recipe, a decrease in the firing frequency may allow most ofthe targets 82 to be irradiated by the pre-pulse laser 42 and the mainpulse laser 52.

In some embodiments, in the cases where the distance D between the firstdetection position DP1 and second detection position DP2 is fixed,operation 93 is omitted, and the measure period of time is compared witha predetermined value associated a desired period of time according to aprocessing recipe. It should be noted that the predetermined valuedescribed above may be refer to as a specific value, a range of value,or multiple ranges of value.

The method 90 also includes operation 97, in which the operating statusof the laser generator LG is monitored to determine if the adjustment ofthe laser generator LG is outside of an acceptable range. In someembodiments, an upper limit firing frequency and a lower limit firingfrequency are set by the controller 13. When the desired firingfrequency associated the calculated flow velocity is within anacceptable range between the upper limit firing frequency and the lowerlimit firing frequency, the controller 13 actuates the adjustment of thelaser generator LG, and the method continues to operation 95.

On the other hand, when the desired firing frequency associated thecalculated flow velocity is outside of the acceptable range between theupper limit firing frequency and the lower limit firing frequency, thecontroller 13 may not adjust the laser generator LG to the desiredfiring frequency, and instead at least one parameter of the fuel targetgenerator 30 is adjusted (operation 98). This is because irradiating thetargets 82 with lasers having a firing frequency that is higher than theupper limit may cause the light source to become contaminated by debris,and because irradiating the targets 82 using lasers with a firingfrequency that is less than the lower limit may lead to a decrease inthe power of the light 84.

In some embodiments, in operation 98, the pressure of the pumping gas 33in the fuel target generator 30 is modified so that the flow velocity ofthe targets 82 from the fuel target generator 30 is adjustedaccordingly. A method for determining the modified pressure of thepumping gas 33 may include calculating difference between the detectedflow velocity or the measured period of time and the predeterminedvalue. The method further includes comparing the calculated differencewith a lookup table (not shown) to determine the amount of pressurerequired to increase or decrease.

In some other embodiments, the frequency of the voltage supplied to theactuator 35 is changed, so that the frequency for generating the targets82 is adjusted accordingly. With such adjustments to the parameters ofthe fuel target generator 30, the targets 82 can be irradiated with thepre-pulse laser 42 and the main pulse laser 52 at the proper angle andenergy, and the power conversion efficiency of the targets to the lightis improved.

It should be appreciated that while operation 98 is performed afteroperations 96 and 97, the embodiments should not be limited thereto. Insome other embodiments, operations 96 and 97 are omitted, and operation98 is initiated right after operation 94 when the flow velocity isdifferent from the predetermined value. Alternatively, operation 96 andoperation 98 can be performed the same time. That is, the conditions ofthe targets 82 are modified to a desired mode by simultaneouslyadjusting parameters of the laser generator LG and the fuel targetgenerator 30.

In some other embodiments, if the adjusted parameter of the fuel targetgenerator 30 is outside of an acceptable range, a warning signal isissued by the controller 13. The warning signal is sent to the fueltarget generator 30 to stop the supply of the targets 82. In addition,the warning signal triggers the operation of warning equipment (such asa warning light or warning ring not shown in figures) to call apersonnel to perform a maintenance process.

The method 90 may be performed before a beginning of a lithographyexposure process. Alternatively, the method 90 may be periodicallyperformed after a given amount of semiconductor wafers are process theby the lithography system 10. Alternatively, the method 90 may beperformed during the lithography exposure process.

FIG. 6 is a diagrammatic view of the light source 12 a in thelithography system, in accordance with some embodiments. In theembodiments shown in FIG. 6, elements that are similar to those shown inFIG. 2 are provided with the same reference numbers, and the featuresthereof are not reiterated in the interests of brevity. Differencesbetween the light source 12 a and the light source 12 include the lightsource 12 a including two monitoring devices 70. In some embodiments,the additional monitoring device 70 is arranged such that the conditionof expanded targets 83 can be detected.

In some embodiments, the additional monitoring device 70 can detect morethan the flow velocity of the expanded targets 83. For example, bycalculating the duration of the reflected light curtain received by theadditional monitoring device 70, the length of the expanded targets 83can be detected. With such additional information about the expandedtargets 83, the expanded targets 83 can be accurately irradiated withthe main pulse laser 52, and higher power conversion can be achieved.

Embodiments of a method for generating light in lithography exposureprocess are provided. Parameters of a light source are adjustedaccording to collected information of the targets during the lithographyexposure process. Therefore, light emission conversion efficiency isenhanced, and contamination of the light emitting system by debris isreduced.

In accordance with some embodiments, a method for generating light inlithography exposure process is provided. The method further includesmeasuring a period of time during which one of targets from a fueltarget generator passes through two detection positions. The method alsoincludes exciting the targets with a laser generator so as to generateplasma that emits light, wherein the operation of exciting the targetswith the laser generator includes: irradiating a pre-pulse laser on thetargets to expand the targets; detecting conditions of expanded targets;and adjusting at least one parameter of the laser generator according tothe measured period of time and the conditions, when the measured periodof time is different from a predetermined value, wherein the parameterof the laser generator which is adjusted according to the measuredperiod of time comprises a frequency for generating a laser forilluminating the targets.

In accordance with some embodiments, a method for generating light inlithography exposure process is provided. The method includes detectinga flow velocity of targets from a fuel target generator at a firstdetection position and a second detection position. The method furtherincludes adjusting the flow velocity of the targets in response to thedetected flow velocity when the detected flow velocity is different froma predetermined value. The method also includes irradiating at least onelaser with a laser generator on the targets to generate plasma thatemits light. In addition, the operation of irradiating the at least onelaser on the targets includes: irradiating a pre-pulse laser on thetargets to expand the targets; detecting conditions of expanded targets;and adjusting a parameter of the laser generator according to thedetected flow velocity and the conditions. Furthermore, the operation ofdetecting the flow velocity of the targets includes: projecting a firstlight curtain and a second light curtain at the first detection positionand the second detection position, respectively, on a moving path;receiving light reflected from one of the targets when the one of thetargets passes through the first detection position and the seconddetection position; measuring a time difference between a time when thelight reflected from the target at the first detection position isdetected and a time when the light reflected from the target at thesecond detection position is detected; and determining a flow velocityof the targets by dividing a distance between the first detectionposition and the second detection position by the measured timedifference.

In accordance with some embodiments, a method for generating light inlithography exposure process is provided. The method further includesmeasuring a period of time during which one of targets from a fueltarget generator passes through two detection positions. The method alsoincludes exciting the targets with a laser generator so as to generateplasma that emits light, wherein the operation of exciting the targetswith the laser generator includes: irradiating a pre-pulse laser on thetargets to expand the targets; detecting length of expanded targets; andadjusting at least one parameter of the laser generator according to themeasured period of time and the length of expanded targets, when themeasured period of time is different from a predetermined value, whereinthe parameter of the laser generator which is adjusted according to themeasured period of time comprises a frequency for generating a laser forilluminating the targets.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture,composition of matter, means, methods, and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method for generating light in a lithographyexposure process, comprising: measuring a period of time during whichone of targets from a fuel target generator passes through two detectionpositions; exciting the targets with a laser generator so as to generateplasma that emits light, wherein the operation of exciting the targetswith the laser generator comprises: irradiating a pre-pulse laser on thetargets to expand the targets; detecting conditions of expanded targets;and adjusting at least one parameter of the laser generator according tothe measured period of time and the conditions, when the measured periodof time is different from a predetermined value, wherein the parameterof the laser generator which is adjusted according to the measuredperiod of time comprises a frequency for generating a laser forilluminating the targets.
 2. The method for generating light in alithography exposure process as claimed in claim 1, wherein theoperation of measuring the period of time comprises: projecting a firstlight curtain and a second light curtain at the two detection positions;receiving light reflected from one of the targets when the one of thetargets passes through the two detection positions; and measuring aperiod of time between a time when the light reflected from the targetat a first detection position is detected and a time when the lightreflected from the target at as second detection position is detected.3. The method for generating light in a lithography exposure process asclaimed in claim 1, wherein the targets generated from the fuel targetgenerator are separated by a droplet pitch, and a distance between thetwo detection positions is smaller than the droplet pitch.
 4. The methodfor generating light in a lithography exposure process as claimed inclaim 1, wherein the two detection positions are located between thefuel target generator and a lighting position where the targets arefocused by the pre-pulse laser.
 5. The method for generating light in alithography exposure process as claimed in claim 1, wherein thepre-pulse laser irradiates the targets to expand the targets at a firstlighting position, and the operation of exciting the targets to generateplasma further comprises: irradiating a main pulse laser on the expandedtarget to generate the plasma at a second lighting position; wherein thetwo detection positions are located between the first lighting positionand the second lighting position.
 6. The method for generating light ina lithography exposure process as claimed in claim 1, further comprisingadjusting a parameter of the fuel target generator according to themeasured period of time, wherein the parameter of the fuel targetgenerator comprises a flow velocity of the targets generated from thefuel target generator.
 7. A method for generating light in a lithographyexposure process, comprising: detecting a flow velocity of targets froma fuel target generator at a first detection position and a seconddetection position; adjusting the flow velocity of the targets inresponse to the detected flow velocity when the detected flow velocityis different from a predetermined value; and irradiating at least onelaser on the targets with a laser generator to generate plasma thatemits light; wherein the operation of irradiating the at least one laseron the targets comprises: irradiating a pre-pulse laser on the targetsto expand the targets; detecting conditions of expanded targets; andadjusting a parameter of the laser generator according to the detectedflow velocity and the conditions; wherein the operation of detecting theflow velocity of the targets comprises: projecting a first light curtainand a second light curtain at the first detection position and thesecond detection position, respectively, on a moving path; receivinglight reflected from one of the targets when the one of the targetspasses through the first detection position and the second detectionposition; measuring a time difference between a time when the lightreflected from the target at the first detection position is detectedand a time when the light reflected from the target at the seconddetection position is detected; and determining a flow velocity of thetargets by dividing a distance between the first detection position andthe second detection position by the measured time difference; whereinthe parameter of the laser generator which is adjusted according to themeasured time difference comprises a frequency for generating a laserfor illuminating the targets.
 8. The method for generating light in alithography exposure process as claimed in claim 7, further comprising:calculating a time interval by dividing an intermediate distance by thedetected velocity, wherein the intermediate distance is between thefirst detection position and a lighting position where the targets areilluminated by the laser; and wherein the laser is actuated after thetime interval when the light reflected from the target at the firstdetection position is detected.
 9. The method for generating light in alithography exposure process as claimed in claim 7, wherein theoperation of irradiating at least one laser on the targets comprises:irradiating a main pulse laser on the expanded target to generate plasmathat emits light; wherein the velocity of the targets is detected beforethe targets are expanded by the pre-pulse laser.
 10. The method forgenerating light in a lithography exposure process as claimed in claim7, wherein the operation of irradiating at least one laser on thetargets comprises: irradiating a main pulse laser on the expanded targetto generate plasma that emits light; wherein the velocity of the targetsis detected after the targets are expanded by the pre-pulse laser andbefore the expanded targets are irradiated by the main pulse laser. 11.The method for generating light in a lithography exposure process asclaimed in claim 7, wherein the step of detecting conditions of expandedtargets includes detecting length of expanded targets.
 12. A method forgenerating light in a lithography exposure process, comprising:measuring a period of time during which one of targets from a fueltarget generator passes through two detection positions; exciting thetargets with a laser generator so as to generate plasma that emitslight, wherein the operation of exciting the targets with a lasergenerator comprises: irradiating a pre-pulse laser on the targets toexpand the targets; detecting length of expanded targets; and adjustingat least one parameter of the laser generator according to the measuredperiod of time and the length of expanded targets, when the measuredperiod of time is different from a predetermined value, wherein theparameter of the laser generator which is adjusted according to themeasured period of time comprises a frequency for generating a laser forilluminating the targets.
 13. The method for generating light in alithography exposure process as claimed in claim 12, wherein theoperation of measuring the period of time comprises: projecting a firstlight curtain and a second light curtain at the two detection positions;receiving light reflected from one of the targets when the one of thetargets passes through the two detection positions; and measuring aperiod of time between a time when the light reflected from the targetat a first detection position is detected and a time when the lightreflected from the target at as second detection position is detected.14. The method for generating light in a lithography exposure process asclaimed in claim 12, wherein the targets generated from the fuel targetgenerator are separated by a droplet pitch, and a distance between thetwo detection positions is smaller than the droplet pitch.
 15. Themethod for generating light in a lithography exposure process as claimedin claim 12, wherein the two detection positions are located between thefuel target generator and a lighting position where the targets arefocused by the pre-pulse laser.
 16. The method for generating light in alithography exposure process as claimed in claim 12, wherein thepre-pulse laser irradiates the targets to expand the targets at a firstlighting position, and the operation of exciting the targets to generateplasma further comprises: irradiating a main pulse laser on the expandedtarget to generate the plasma at a second lighting position; wherein thetwo detection positions are located between the first lighting positionand the second lighting position.
 17. The method for generating light ina lithography exposure process as claimed in claim 12, furthercomprising adjusting a parameter of the fuel target generator accordingto the measured period of time, wherein the parameter of the fuel targetgenerator comprises a flow velocity of the targets generated from thefuel target generator.
 18. The method for generating light in alithography exposure process as claimed in claim 12, wherein the lasergenerator comprises: a first laser source configured to generate thepre-pulse laser for expanding the targets at a first lighting position;wherein the two detection positions are located between the fuel targetgenerator and the first lighting position.
 19. The method for generatinglight in a lithography exposure process as claimed in claim 12, whereinthe laser generator comprises: a first laser source configured togenerate the pre-pulse laser for expanding the targets at a firstlighting position; and a second laser source positioned farther awayfrom the fuel target generator than the first laser source andconfigured to generate a main pulse laser for exciting the expandedtargets at a second lighting position; wherein the two detectionpositions are located between the first lighting position and the secondlighting position.
 20. The method for generating light in a lithographyexposure process as claimed in claim 12, wherein the period of time ismeasured by a monitoring device, and the length of expanded targets isdetected by another monitoring device.